package algorithm;

//--------------------------------------
//Systematically generate combinations.
//--------------------------------------

import java.math.BigInteger;

public class CombinationGenerator {

private int[] a;
private int n;
private int r;
private BigInteger numLeft;
private BigInteger total;

//------------
// Constructor
//------------

public CombinationGenerator (int n, int r) {
 if (r > n) {
   throw new IllegalArgumentException ();
 }
 if (n < 1) {
   throw new IllegalArgumentException ();
 }
 this.n = n;
 this.r = r;
 a = new int[r];
 BigInteger nFact = getFactorial (n);
 BigInteger rFact = getFactorial (r);
 BigInteger nminusrFact = getFactorial (n - r);
 total = nFact.divide (rFact.multiply (nminusrFact));
 reset ();
}

//------
// Reset
//------

public void reset () {
 for (int i = 0; i < a.length; i++) {
   a[i] = i;
 }
 numLeft = new BigInteger (total.toString ());
}

//------------------------------------------------
// Return number of combinations not yet generated
//------------------------------------------------

public BigInteger getNumLeft () {
 return numLeft;
}

//-----------------------------
// Are there more combinations?
//-----------------------------

public boolean hasMore () {
 return numLeft.compareTo (BigInteger.ZERO) == 1;
}

//------------------------------------
// Return total number of combinations
//------------------------------------

public BigInteger getTotal () {
 return total;
}

//------------------
// Compute factorial
//------------------

private static BigInteger getFactorial (int n) {
 BigInteger fact = BigInteger.ONE;
 for (int i = n; i > 1; i--) {
   fact = fact.multiply (new BigInteger (Integer.toString (i)));
 }
 return fact;
}

//--------------------------------------------------------
// Generate next combination (algorithm from Rosen p. 286)
//--------------------------------------------------------

public int[] getNext () {

 if (numLeft.equals (total)) {
   numLeft = numLeft.subtract (BigInteger.ONE);
   return a;
 }

 int i = r - 1;
 while (a[i] == n - r + i) {
   i--;
 }
 a[i] = a[i] + 1;
 for (int j = i + 1; j < r; j++) {
   a[j] = a[i] + j - i;
 }

 numLeft = numLeft.subtract (BigInteger.ONE);
 return a;

}


/*
public static void main(String[] args)
{
	String[] elements= {"a","b","c","d","e"};
	int[] indices;
	CombinationGenerator x= new CombinationGenerator (elements.length, 2);
	StringBuffer combination;
	while(x.hasMore())
	{
		combination= new StringBuffer();
		indices= x.getNext();
		for(int i=0; i<indices.length; i++)
		{
			System.out.println("elements[indices[i]] " + elements[indices[i]]);
			combination.append(elements[indices[i]]);
		}
		System.out.println("combination " + combination.toString());
	}
}

*/


}
